PHYSIOLOGY OF NERVE TISSUE. 73 



PHYSIOLOGIC PROPERTIES OF NERVES. 



Nerve Irritability or Excitability and Conductivity. These terms are 

 employed to express that condition of a nerve which enables it to develop and 

 to conduct nerve impulses from the center to the periphery, from the periphery 

 to the center, in response to the action of stimuli. A nerve is said to be ex- 

 citable or irritable as long it possesses these capabilities or properties. For the 

 manifestation of these properties the nerve must retain a state of physical and 

 chemic integrity; it must undergo no change in structure or chemic compo- 

 sition. The irritability of an efferent nerve is demonstrated by the contrac- 

 tion of a muscle, by the secretion of a gland, or by a change in the caliber of 

 a blood-vessel, whenever a corresponding nerve is stimulated. The irrita- 

 bility of an afferent nerve is demonstrated by the production of a sensation or a 

 reflex action whenever it is stimulated. The irritability of nerves continues 

 for a certain period of time after separation from the nerve centers and even 

 after the death of the animal, varying in different classes of animals. In the 

 warm-blooded animals, in which the nutritive changes take place with great 

 rapidity, the irritability soon disappears a result due to disintegrative 

 changes in the nerve, caused by the withdrawal of the blood-supply. In cold- 

 blooded animals, on the contrary, in which the nutritive changes take place 

 relatively slowly, the irritability lasts, under favorable conditions, for a con- 

 siderable time. Other tissues besides nerves possess irritability, that is, the 

 property of responding to the action of stimuli e. g., glands and muscles, 

 which respond by the production of a secretion or a contraction. 



Independence of Tissue Irritability. The irritability of nerves is 

 distinct and independent of the irritability of muscles and glands, as shown 

 by the fact that it persists in each a variable length of time after their histo- 

 logic connections have been impaired or destroyed by the introduction of 

 various chemic agents into the circulation. Curara, for example, induces a 

 state of complete paralysis by modifying or depressing the conductivity of the 

 end organs of the nerves just where they come in contact with the muscles 

 without impairing the irritability of either nerve trunks or muscles. Atropin 

 induces complete suspension of glandular activity by impairing the terminal 

 organs of the secretor nerves just where they come into relation with the gland 

 cells, without destroying the irritability of either gland or nerve. 



Stimuli of Nerves. Nerves do not possess the power of spontaneously 

 generating and propagating nerve impulses; they can be aroused to activity 

 only by the action of an extraneural stimulus. In the living condition the 

 stimuli capable of throwing the nerve into an active condition act for the most 

 part on either the central or peripheral end of the nerve. In the case of mo- 

 tor nerves the stimulus to the excitation, originating in some molecular dis- 



